Don't be naive. If McCabe expected the Prince to put a certain amount of money in, he should have made it a contractual obligation. If the Prince didn't do so, McCabe should have been able to break the arrangement. Doing anything with a business partner based on trust - especially when there are millions of pounds involved - is misguided at best.
If he'd suggested it and the Prince had said no, he could have taken that as evidence of the latter's lack of serious interest and told him to jog on.