Can we borrow a player from a club in another province?

Not club to club. The SARU Player Status, Player Contracts and Player Movement Regulations say players may not be loaned between clubs in different provinces (7.5). Loans are between provinces:

• The borrowing province asks the player's current province in writing, with the player's written approval, and the terms go into the standard Player Loan Agreement (Schedule IV). The current province must reply in writing within 14 days.
• A loan player stays registered with their club in the current province and does not register with a club in the borrowing province.
• The player may only play in the competitions named in the loan agreement.
• A player who was in any provincial 22-player squad that season may not be loaned to a club in another province unless both provinces agree in writing.
• Provinces must keep a record of all loan players and give copies to SARU on demand.

In SA Rugby provincial competitions, a loan player may not be in the borrowing province's match-day squad without a written loan agreement between the two provinces and the player.