If it can be proved that a verbal agreement was made it’s as legal as a written agreement, obviously if there were no witnesses it’s hard to prove that a verbal agreement was made and the employer would be a bit daft to take it any further.
Having said that, you took the bosses money so why not now stick to the agreement you had, when people [zb] on these sort of agreements it just makes employers less likely to help anyone else by paying for the training.