It is not permissible to buy the note which is mentioned in the question, i.e. the seven hundred thousand riyals promissory note for a cash sum of three hundred thousand riyals. This is because doing so is Riba which is Haram. Allah (Exalted be He) says:
O you who believe! Be afraid of Allâh and give up what remains (due to you) from Ribâ (usury) (from now onward), if you are (really) believers.
And if you do not do it, then take a notice of war from Allâh and His Messenger
(Part No. 13; Page No. 323) Moreover, it is authentically reported that the Prophet (peace be upon him):
cursed is the devourer (taker) of Riba, its payer, its scribe and its two witnesses. He said, 'They are all equal.'
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.