It's possible to make an ability incompatible with itself.
EDIT: Also is it possible to change the date order? Having it YMD or MDY is offputting for myself as a Brit.
EDIT2: Stray thought, I know in the personnel market we're probably paying the unit cost but it'd be nice to see that listed next to the person (perhaps with the variance of the unit markets in pricing, for example a recruit offered by your employer, etc).
EDIT3: Day progression isn't working properly, which is also breaking payment by contracts.
Abilities *can* be made incompatible, pre-requisite or self-removing when editing them. Not sure that fixing that is going to make it into the stable, as it's a relatively minor issue that's easily worked around by not doing it.
Date format changes are, I believe, being worked on. However, that's a long-term project that's not going to come out for the upcoming stable.
When you hire someone from the personnel market, you're not paying anything up front. It's more like hiring someone for a job.
You're going to have to be more specific about day progression. Without details, we can't really help you out. I would suggest going on
https://github.com/MegaMek/mekhq/issues and posting your campaign save, game version and what exactly you're doing.