If the co-owner of house voluntarily leaves is he still responsible for costs to maintain and/or sell?

2 single people own a house as TIC. Both are listed on the deed and mortgage. 1 leaves voluntarily. The individual that left (Owner B) verbally agrees to pay an agreed upon amount to owner A when the house is refinanced or sold, but does not contribute to any current costs related to the house. The owner still in the house (Owner A) is unable to sell or refinance due to market conditions and the low amount of equity in the house and is forced to keep the house afloat for multiple years. When the market recovers, the house is sold for an amount less than the original purchase price. The small amount of equity Owner A built in the house covers most of the sale costs (commissions, etc), but the owners receive no cash from the sale. Owner B reneges on his offer to pay an agreed upon amount to owner A. Is Owner B responsible for half of the costs (mortgage, property taxes, maintenance) associated with maintaining the house until a sale was feasible? In addition, is owner B responsible for a percentage of the closing costs in relation to the amount of equity he/she was responsible for building in said property? In short, can owner A take the original agreed upon offer off the table and pursue a greater amount? I had to choose "divorce" as Civil wasn't an option, but the individuals were not married.