His father is unable to give his wife the dowry (mahr); can the son give it on behalf of his father?
Question

My father couldnt give dowry(mahr) to my mother because of some reason, and my father is do not work as he retired from the company, can a son give the amount to his father. And offer my mother as dowry (Mahr)?.
Praise be to Allah.Firstly: 
The dowry is the wife’s right over her husband. If the husband names a dowry for his wife, then he is not able to pay it, then it is a debt he owes like any other debt, unless the woman waives it willingly, in which case the husband is no longer liable for it, because it is the woman’s right and she has waived it. Allah, may He be exalted, says (interpretation of the meaning): “And give to the women (whom you marry) their Mahr (obligatory bridal money given by the husband to his wife at the time of marriage) with a good heart, but if they, of their own good pleasure, remit any part of it to you, take it, and enjoy it without fear of any harm (as Allah has made it lawful)” [an-Nisa’ 4:4]. 
Ibn Qudaamah (may Allah have mercy on him) said: If the woman foregoes her dowry which is her due from her husband, or she foregoes part of it, or gives it to him after taking possession of it, that is permissible and valid, and we do not know of any scholarly difference of opinion concerning it, because of the verse in which Allah says (interpretation of the meaning): “unless they (the women) agree to forego it (the mahr)” [al-Baqarah 2:237]. And Allah says (interpretation of the meaning): “ … but if they, of their own good pleasure, remit any part of it to you, take it, and enjoy it without fear of any harm (as Allah has made it lawful)” [an-Nisa’ 4:4].
End quote from al-Mughni, 7/196 
The scholars of the Standing Committee (19/196) were asked: Is the mahr (dowry) regarded as a debt that must be paid off or not? 
They replied: Whatever dowry is named to the wife becomes due to the wife upon consummation or if the husband dies, and half of it must be given to her if she is divorced before consummation. In either case, what she is entitled to is a debt for which the husband is liable and which must be paid off, unless she willingly remits all or part of it, in which case it is waived. Allah, may He be exalted, says (interpretation of the meaning): “And if you divorce them before you have touched (had a sexual relation with) them, and you have appointed unto them the Mahr (bridal money given by the husbands to his wife at the time of marriage), then pay half of that (Mahr), unless they (the women) agree to forego it, or he (the husband), in whose hands is the marriage tie, agrees to forego and give her full appointed Mahr. And to forego and give (her the full Mahr) is nearer to At-Taqwa (piety, right-eousness, etc.). And do not forget liberality between yourselves. Truly, Allah is All-Seer of what you do” [al-Baqarah 2:237]. And He says (interpretation of the meaning): “And give to the women (whom you marry) their Mahr (obligatory bridal money given by the husband to his wife at the time of marriage) with a good heart, but if they, of their own good pleasure, remit any part of it to you, take it, and enjoy it without fear of any harm (as Allah has made it lawful)” [an-Nisa’ 4:4].
End quote. 
Standing Committee for Academic Research and Issuing Fatwas 
Shaykh ‘Abd ar-Razzaaq ‘Afeefi, Shaykh ‘Abdullah ibn ‘Abd ar-Rahmaan ibn Ghadyaan, Shaykh ‘Abdullah ibn Sulaymaan ibn Munayyi ‘ 
Secondly: 
It is permissible for the son to pay off a debt on behalf of his father, whether it is a dowry or a loan, and whether the father is incapable or otherwise. But if he is incapable, in this case it is permissible for the son to pay off his debt even if that is done using zakaah funds. 
For more information please see the answer to question no. 39175 
And Allah knows best.