Yes, they do pay less tax, they prefer to donate to charity than donate more to gahmen lor, so long they pay less regardless of quantum, that amt saved is considered donated to charity not gahmen lor, hahaha
If a large amount is donated, the donor might get some recognition e.g. building named after them, plaque, media reports...
If they don't donate and pay the tax, the gahmen just thanks them for their contribution to nation building (same as everybody else)