Is calculating a certain percentage—interest—on a sum loaned to a sister company, with the sum and interest to be repaid after one month, considered riba (usury) or murabaha (cost-plus financing)?

This is an explicit usurious loan, not a murabaha sale, and it is forbidden, as Allah Almighty says: "But Allah has permitted trade and forbidden interest." It is not permissible to do this, nor to cooperate with its perpetrator, for Allah Almighty says: "O you who have believed, fear Allah and give up what remains [due to you] of interest, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger," and "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." The Prophet, peace and blessings be upon him, cursed the one who consumes interest, the one who pays it, its scribe, and its two witnesses, and he said: "They are all equal." The employer must know that what it calculates as interest is pure usury, and it must desist from it, and it has no right to demand from the borrower anything more than its principal capital without any increase.