Conditional trading, allowed?

What are your thoughts on trades that include conditions? Should they be allowed? I feel uneasy but don't have a logical reason to disallow them. Here are some examples:

A for B, and if A gets injured after the trade, the trade is reversed

A for B, if A or B gets injured after the trade, the trade is reversed

Team 1 and Team 2 have players on byes alternating weeks so they trade for one week and then trade back, effectively borrowing players.