I think you’re in a better negotiating position than you think here OP, depending on a few things, and if you’re good at bluffing/negotiationg.
Did your ex ever live in the property you’re selling? For how long? Where did money for this purchase come from?
How many years have you been separated?
I don’t know if you can put a charge on a property you have never lived in, but I do know ex won’t be able to claim on yours without you claiming on his, or for that to minimally be a mitigating factor.
Are the two properties similar in value?
Scenario 1- ex claims in interest in your property to current date value because you’re still married, which also means he has to accept you have a marital assets claim on his share of the inherited property to date.
Scenario 2 - he claims you have no interest in the inherited property because it was inherited post-separation, which means you will be able to at least discount his interest in your property by X years. He can’t have it both ways.
Scenario 3 - you both agree to release the other from a claim against properties in exchange for a % of each property. Be great if his was worth more as then this won’t be appealing at all.
Hold your nerve and negotiate quickly and calmly.
Is he up to date on CM?