Was it permissible for a mujahid during the time of Sayyiduna Umar ibn al-Khattab to take a loan from the Caliph for a certain amount and repay it with an increase, on the basis that the stipends of the mujahidin were grants from the ruler, which he was allowed to dispose of?

Usury issued by the state is more sinful and criminal than individual usury because it is legalized usury. What the questioner assumed about Umar ibn al-Khattab and the mujahideen dealing with usury is incorrect. They were extremely pious and scrupulous in this regard, as evidenced by his reluctance to accept a gift from Ubayy ibn Ka'b, to whom he owed money. What the questioner mentioned about the issue of grants and salaries is incorrect, for Umar ibn al-Khattab, may Allah be pleased with him, was the first to establish diwans (bureaus) to distribute salaries to their rightful recipients according to their precedence in Islam.