[2019] Redeemed a bond to sell on GE and got membership instead. Is there a way to undo this as I have over 40 days of membership and 14 days that I do not need.

There's no harm in asking the Support team, I'm unsure whether they'd accommodate it since you technically used the bond (but not the membership).

Try this route: https://support.runescape.com/hc/en-gb/articles/360000982997. It's for refunds, which is pretty accurate since you want the bond refunded.

Be as clear and concise as possible (you want the membership removed and the bond replaced).