I generally agree with Dzug's answer i.e. any agreement for the sponsors to pay up or for the sponsored person to pursue the sponsorship money is probably binding in honour only (i.e. it there is no contract).
The only exception might be where the person who is sponsored gains in some way from the agreement to get sponsorship. For example, some charities offer people a 'free' parachute jump to people who guarantee to get a minimum amount of sponsorship. In such cases, it would be reasonable to assume that the person making the parachute jump had entered into a contract to make reasonable attempts to pursue payment of the money which was promised by the sponsors (including those amounts which would bring the total collected to above the minimum level). However, as long as reasonable attempts at collection were made, the sponsored person would not held liable for any non-payment by sponsors as long as the minimum amount was paid to the charity. (The sponsored person is usually obliged to sign a contract guaranteeing that, in return for the jump, he/she will guarantee the minimum amount). Even so, the sponsors would seem to be bound by nothing more than honour.
Chris