Does an increase in salary for extra work, agreed upon after its completion, fall under the category of Riba an-Nasi'ah (interest due to delay)? And how should one dispose of this money if that is the case?

Usury of Nasi’ah (delay) is the delay in taking possession of one of the two counter-values in the sale of usurious commodities that share the same underlying cause, such as selling gold for gold or currency for currency. In such transactions, immediate possession in the contracting session is required, based on the Prophet’s (peace and blessings be upon him) saying: “Gold for gold… hand to hand.” The described transaction does not fall under the usury of Nasi’ah, and the money earned from it is permissible if it is with the consent of the authorized person responsible for disbursing company funds.