Is it permissible to collect donations to purchase a plot of land and build a home for marriage, with the intention of donating an equivalent amount when able?

The basic principle regarding fundraising through websites is prohibition, unless there is an urgent need. If the websites are specifically designated for this purpose, and the solicitor deals with truthfulness and honesty, then there is no harm upon him. This is not part of the blameworthy asking, but rather falls under informing about one's situation to those who have dedicated themselves to assistance, as Sheikh Ibn Uthaymeen said.