Can a Cop Make me Pay Money?

Ok so me and my friend were on our way to my house and we found a little dog white it was mixed with something had poka dots on its eyes (Owners Claim Pur bread Maltease) Anyways i dont have a cell and there was a tag on the dog with a number and my friend called them and suposivley they let the dog out and diddent want it anymore and i cant have anymore dogs nor can she so we got rid of it 6 days later cops come to my house saying wheres the dog i told them excally what happen and they said i have to either give them 500 dollars to the dog owners and so does my friend 1000 all togther or goto jail they wouldent let me fill out a witeness report or anything the cop saw no proof its a purbread but i guess he is takin there word and they showed me no proof its a pur bread and i did nothing wrong i was told they diddent want it so bottom line is my friend lied i was only helping her get rid of it so now what? and im 16 i dont wanna goto jail i collect SSI for a living im in foster care :(