You should retain an attorney to help you resolve this. The short answer is, IF you signed a settlement agreement stating that you were settling any and all liability connected with the accident for $2,500, that agreement would be enforceable. Once someone settles a given claim in full, for a certain amount, the other party cannot then seek additional money (though other persons injured or damaged could do so). Without a settlement agreement, you would be in much worse shape, since in that case,  there was, literally, no agreement to take $2,500 as settlement in full. If there was no settlement agreement, the situation is simply that of someone asking you for money and you givinig it to them, without getting anything in return. The amount you paid should be credited vs. the full amount you may end up owing, but that's about it. Note that you do NOT have to take the other side's word for (1) that you are responsible for the accident (though you do seem to admit that you were); or (2) the extent of a damages or costs--you can make them prove it and dispute anything you disagree with. An attorney can help you do this.