Is buying a share for $1500 from a foreign company, with a guaranteed capital and a return of $500 monthly for one year (totaling $6000), considered clear usury, or is it permissible to receive money from non-Muslims?

This transaction is usurious (riba) and impermissible because it is a loan with an increase. A loan with an interest is the very essence of usury. The Sharia texts that prohibit usury do not differentiate between Dar al-Harb (House of War) and Dar al-Islam (House of Islam), nor between a Muslim's transaction with another Muslim or with a non-Muslim. It is forbidden to appropriate the wealth of non-Muslims if they are Dhimmis or Musta'mins.