I've read a lot of posts about amending prior years tax returns, and filing a form 3115, but I'm still unclear on if it's possible to retroactively take 100% bonus depreciation. 

I bought a business where the prior owner was sued for $3 million causing injuries to some customers.

While buyers and their agents have always had the option to enter into a written agreement, it is now mandated that this happen prior to touring a home.

I love having a video/photo inspection prior to move in for my records.

My wife and I opened a Chicago Style hot dog cart, and I reprised my handyman business I started prior to COVID.

All of this would be written out in an addendum to the sales contract and signed by both parties prior to closing. 

It went from 85% purchase price covered to 65% even after the deal had been analyzed by them extensively prior.

Was the property in service in the prior year? 

There's nothing preventing you from looking at replacements properties prior to closing on the property you're selling, You can even be under contract before you even close on your relinquished property.

Louisville MSD just increased rates 7% again and this year we are averaging $700/month between water+sewer bill which has basically bankrupt the association as all prior years were budgeted for half of this.