To start, I know that this has been said before, but this is a slight change on it.
For growing airlines- especially newer ones, where margins and cash reserves are tight, having funds tied up in the lease market is a significant disadvantage if someone outbids you. To combat this, I think getting your money back would be a benefit. I know about the cheating possibilities so why not just limit the number of times that you can get your money back early? Wouldn’t this essentially fix or severely limit the ramifications of price bumping?
Perhaps limit it to 2 lease deposits returned per 24 hour period. You could have a timer and status saying something like “1 out of 2 returnable lease deposits remaining until 23:45 UTC” at the top of the aircraft market screen. This would allow players get their more critical deposits returned sooner, but prevent players from just bidding on aircraft that they don’t care about en masse. Heck, even if a player just got one per 24 or 48 hours, that would be very significant and could get them out of an extremely long bidding process.