Is it possible to have "an after the fact" agreement with your spouse ?

Asked in Fort Lauderdale, FL - about 1 year

me and my husband been together in Marriage for the past 2 years during that time we discussed many times about having our own Biasness and now after Me and my 4 years old left the house (there is an alcohol problem, domestic violence) he calls and after threatening to make up stories and said he will make sure he will do anything is his power to make sure I won’t get my permanent green card, since I have my green card over a year now, he said he is “willing” to make sure I’ll get it only if we will do a prenuptial agreement .. Sounds to me like blackmailed.. What should I do...I can’t afford a lawyer and I don’t know where to turn