If a lady is getting married and she has a large sum of money that is her own, is there a halachic way of keeping that money from her husband so that he won’t have any rights to it (in case the marriage does not end up working out)?
The only way of ensuring that the husband has no rights to the money is to either 1) make an explicit agreement with the future husband (not recommended, because showing a lack of trust before the wedding is not a recipe for success), or 2) making a full legal transfer of the money to somebody else.
For the second option, the legal transfer must be true, and not a fiction. However, clauses can be inserted to ensure that the money will be readily available for the lady in question’s use.
Note that even if nothing is done, a wife’s money (from before the wedding) does not transfer to the husband. However, he does receive the basic rights (under regular circumstances) to the ‘fruit’ that grow from the money, such as interest on investments, and so on.