What is the legitimate ruling on a sudden increase in the price of sacrificial animals after the initial agreement, and is it permissible to collect the remaining amount from another source to cover the deficit, given the hardship on the original buyers?

The agent is responsible for the price difference and must pay it to the seller, because he acted without referring to his constituents. Thus, he was negligent. What he paid of the extra price was not with the intention of participating in the Udhiyah, so he does not share in it with them, nor does it affect its validity. Rather, he bought it and slaughtered it with the intention of Udhiyah on their behalf.