Here's a study that indicates the form of the payment is what really matters. When people are offered money for donation, they are less interested in donation; however, an offer of a voucher of similar value doesn't have the same negative effect. In a survey of 467 donors:
A substantial share of respondents declared they would stop being donors if paid a small amount of cash, but we do not find such effects when a voucher of the same nominal value is offered instead. The aversion to direct cash payments is particularly marked among women and older respondents, while there are neither gender nor age differences in the response to the voucher.