Help keep Ivy in a good home

This poor little puppy has had a really rough time, she was rescued from a doorstep where she had been locked outside in the cold for almost an hour crying traumatically! The owner said he couldn't handle her crying all the time as he is not sleeping and handed her over and asked us to bring her back when she's behaving! She was taken to the vets and diagnosed skeletal and riddled with worms. The vets advised she should not be taken back to the owner or breeder as she had clearly been mistreated. She has been living with us for 9 days now and has really become attached to the family, has recovered from the worms and is starting to put on some weight. We have had advice from the RSPCA and all they have said is that the legal owner is the man who bought her. We have been back in contact with him, he does not want her but wants to resell her, he is demanding £1000 immediately if we want to keep her. Please help us keep her here where she is happy and settled and getting all of the love and attention that she needs.