When should a plaster cast be taken off? If he knew that the limb was healed before taking it off, does he have to repeat the prayer?
Question

I had a plaster cast on my left arm, and I was travelling. I did wudu’ and wiped over my left arm, because it had a cast on it, and I prayed Zuhr and ‘Asr, shortening them and putting them together. Straight after praying, I took off the plaster cast and I felt that my arm was not hurting me, then I realised that I could have washed it if there had been no cast on it. Do I have to repeat the two prayers, and should I shorten them or offer them in full?
Praise be to Allah.Firstly:
A plaster cast is something that is placed over a broken limb to keep it straight whilst it heals. In Islamic jurisprudence, the Malikis use the word translated here as plaster cast in a more general sense, referring to anything that is used to treat a wound, such as a splint, dressing and so on.
End quote from al-Mawsu`ah al-Fiqhiyyah, 15/106.
The jurists are unanimously agreed that it is prescribed to wipe over plaster casts when there is a reason to do so.
Ibn Qudamah (may Allah have mercy on him) said: There are five differences between wiping over a plaster cast and wiping over the khuffs, one of which is that it is not permissible to wipe over it unless removing it will result in harm, and this does not apply in the case of khuffs.
End quote from al-Mughni, 1/312.
In al-Mawsu`ah al-Fiqhiyyah, 15/108, it says: The conditions for wiping over a plaster cast:
a.. If washing the broken or injured limb will harm it. The same applies if wiping the injury site directly would harm it, or there is the fear of harm resulting from removing the plaster cast. End quote.
Shaykh Abu `Umar ad-Dibyan (may Allah preserve him) said: One of the conditions of it being permissible to wipe over a plaster cast is that washing would cause harm to the broken limb or infected wound, or there is the fear that some other kind of harm would result from removing the cast.
But if washing will not cause any harm, and there is no fear of harm resulting from removing the cast, then it must be washed, because washing the limb is obligatory, but it may be replaced by something else, namely wiping, when there is a reason for that. But if there is no reason or excuse, washing the limb is not waived. There is no difference of scholarly opinion regarding that.
End quote from Mawsu`at Ahkam at-Taharah, 5/619.
Secondly:
As for how long the plaster cast should remain, reference should be made to a doctor or experts. If experts state a specific time period and you did not go beyond that, then what appears to be the case is that there is no blame on you.
But you must take off the cast when that time period ends, and it is not valid for  you to wipe over it after that.
Shaykh Muhammad ibn Muhammad al-Mukhtar ash-Shinqiti (may Allah preserve him) said: “Until he removes it” means: until he removes the bandage or plaster cast, but he should refer to experts regarding that. If the doctors say that he must keep the cast on for a month, then its time limit is no more than one month. If they say that he must keep it on for two months, then the same applies, and he should not keep it on for longer than the time that the doctors have said is necessary. If he keeps it on for longer than that, then it is not valid for him to wipe over it during that additional time.
End quote from Zad al-Mustaqni`, 14/21.
If the experts did not state a time period, and you think it most likely that you wiped over it after it had healed, then your wiping over it and your wudu’ were not valid, and you must repeat the prayers that you offered.
An-Nawawi (may Allah have mercy on him) said: If what is under the cast had healed and he was not aware of it, and he offered some prayers after that, he must make them up, and there is no difference of scholarly opinion regarding that. The author of at-Tatimmah and others narrated that there was consensus on that.
End quote from al-Majmu`, 2/332.
If you repeat those two prayers when you are not travelling, you must offer them in full, to be on the safe side.
It says in al-Mawsu`ah al-Fiqhiyyah, 27/281: The Hanafis, Malikis and Shafa`is, according to their earlier opinion, said: whoever misses a prayer when travelling should make it up with two rak`ahs when he is not travelling, and whoever misses a prayer when he is not travelling  should make it up when he is travelling with four rak`ahs, because making up the prayer is connected to what you were supposed to do…
The Shafa`is, according to their later view – which is the more correct view – said that it is not permissible for him to shorten the prayer, because shortening the prayer is a concession that is connected to a reason, and that reason no longer applies.
If he missed the prayer when travelling and makes it up when travelling, there are two views, one of which is that he should not shorten the prayer, because it is a prayer that was reduced from four rak`ahs to two, and one of its conditions was that it should have been offered on time.
The second view is that he may shorten the prayer – and this is more correct – because it is a concession that is connected to a reason, and the reason is still in effect, so the concession remains.
If he missed a prayer when he was not travelling, and he makes it up when he is travelling, it is not permissible for him to shorten it, because what he owes is a complete prayer. Therefore it is not permissible for him to shorten it. However, al-Muzani said that he may shorten it.
The Hanbalis said: If he forgot a prayer when he was not travelling, then remembered it when he was travelling, or he forgot a prayer when he was travelling, then remembered it when he was not travelling, in both cases he should offer the prayer of one who is not travelling [i.e., offer it in full]. This was stated by Ahmad in a report narrated by Abu Dawud and al-Athram. That is because shortening the prayer is one of the concessions of travel, and that concession ceases when the reason for it no longer applies. End quote.
And Allah knows best.