Is the husband sinful for not paying his divorced wife the full agreed-upon mahr (thirty thousand), and sufficing with paying five thousand instead? And does the wife have the right to demand the full amount from him before Allah, especially since the husband did not fall short in spending on her during the marriage, and the agreement on the mahr was hasty?

If the agreed upon dowry at the time of the marriage contract was thirty thousand, then the wife is entitled to it upon consummation or valid seclusion, even if she was previously divorced. Giving the wife five thousand does not absolve the husband of his responsibility, and he must pay her the remaining dowry, even if in installments according to his ability, unless she waives it.