Union will not accept payment agreement

I received a phone call on 15 July to advise me of a claim for a hospital visit for May 2019 for R 8 530 I asked the lady why I am only hearing about this claim 2 years after the fact she couldn't give me an answer I told her I would come back to her. On 30 September I phoned back and spoke to someone else and I advised him that we would pay them the full amount by 15 December and he took note of that. On 20 October I received a e-mail that stated if I don't pay half of the amount they will hand me over the more I kept telling him I don't have money to pay but he wouldn't stop asking me to pay half.