The idea behind getting well-known names involved is not for their money but to increase awareness. This encourages people to donate to the charity. When Indian cricketers get involved, it brings in donations from Pakistanis who probably feel - if they can, why can't we, this charity must be genuine and sincere.
Yuvraj and Harbhajan are probably regretting getting involved with the cause, and not the pennies (by their standards) they donated.
If Afridi were to return their money, it would be a third-rate act because how would that cover the goodwill they generated at the time. Or is money the only goodwill that counts?