Perhaps Ash would not mind and would wait for him to get back to normal before he could pay it back but he was not shameless enough to forget about such a debt.
It would always be in the back of his mind that he needed to pay it off and the moment that he accepted too much, he worried that his relationship with Ash would be affected if he could not pay her back right away.
Ash may have said that it was fine like that but he did not feel that it would not have some negative effect on their relationship.
There would surely be a time when Ash needed the money back if she ran into trouble but if he could not give her it back then that kind of situation would make things quite awkward between them both.
Despite his fears, he had to still think about the future and how he was going to overcome such a crisis so he accepted a little help from her even if it was a little above what he had previously called for.