It is not permissible to add more to the price in return for adding something to the goods after agreeing on the price, unless that is done with the consent and knowledge of the purchaser
Question

                I work in admin/accounts position in a stone factory. we get huge orders from customers. we initially give them a price say for 209 pieces price is $1000 but when actually shipping it we find that there are 210 pieces so we increase the price by another say $5 and make it $1005 which is a small amount and client usually do no complain. since $1000 was initially agreed then we later add $5 and also put an extra piece, is this allowed? am i doing right thing?.
Praise be to Allah.
      If the customer accepts the increase of five dollars in the 
      price in return for an increase in the number of pieces, or it is known 
      from his usual attitude in transactions with you that he will accept it, 
      there is nothing wrong with that, because the condition of mutual consent 
      between both parties is one of the conditions of the sale being valid, and 
      that applies in your case. 

    But if he does not accept it, it is not permissible to add 
    the extra piece to what he has purchased from you in return for adding 
    something to the price. Rather if you discover that this addition has been 
    made, then what is required is to give him the choice between sending it in 
    return for the amount mentioned or taking it away from the total that 
    originally agreed upon, if you agree to that. 

    Allah says (interpretation of the meaning):

    “O you who believe! Eat not up your property among 
    yourselves unjustly except it be a trade amongst you, by mutual consent”

    [al-Nisa’ 4:29]. 

    Ibn Maajah (2185) narrated that Abu Sa‘eed al-Khudri (may 
    Allah be pleased with him) said: The Messenger of Allaah (blessings and 
    peace of Allah be upon him) said: “Transactions may only be done by mutual 
    consent.” 

    Classed as saheeh by al-Albaani in Saheeh Ibn Maajah. 

    If the transaction is not done on the basis of agreement, 
    then it comes under the heading of a forced transaction, and is not valid. 

    Zakariya al-Ansaari said in Asna’l-Mataalib (2/6): 

    Forced sales are not valid because of the verse 
    (interpretation of the meaning): “Eat not up your property among 
    yourselves unjustly” [al-Nisa’ 4:29] and because of the report: 
    “Transactions may only be done by mutual consent.” End quote. 

    Shaykh al-Islam (may Allah have mercy on him) said:  

    “A forced sale for which there is no legitimate reason is not 
    binding, according to the consensus of the Muslims. End quote. 

    Majmoo‘ al-Fataawa, 29/199 

    The same applies when buying, as was stated in 
    al-Istiqaamah, 2/321 

    And Allah knows best. 

    See also the answer the questions no.
    1847 and
    129847.