H1B’s My Ex-company is asking pay $10000 for experience letter-- help/suggestion needed

I am seeking your advice/suggestion regarding below problem I have with my previous company.

I am on H1B visa , brought from India to work in USA(VA) for ex-company’s clients here in US.

I have switched to some other employer 6 months back as my project was ending and there was tiny chances that my stay in US would be extended.I feared that I would be forced to go back India.However ex-company gave me an offer letter with conditions like I will have to serve 3 months notice period or pay $10000 in case I switch the company. I wanted to know if such conditions are legally valid here in US? Now for GC processing my current company asked for detail experience letter from of ex-company which they declined to give unless I pay $10000. I signed digitally the offer letter after arriving to USA and later came to know that this kind of agreement is not valid in USA.After leaving the company I have made my best effort to negotiate and was ready to pay up to $4000 best on my ability, but they were not ready to negotiate.I have spent almost a decade (9.5 yrs) in ex-company. Wanted to know if there is anyway I can get the experience letter that is required for Green Card Processing by avoiding paying this money ? Any help would be really appreciated.

I have posted the answer on your wall. Also please delete company name from your post as it will start showing up on google…

Thanks for replying. I have already tried to negotiate with them but they said until I clear my due they won’t provide my documents. and $10000 is a big money, I don’t think I can pay $10000, but they are not in a mood to negotiate. Already two times I have tried to talk to them and asked for a settlement.But all efforts were in vain. What can be done ?