What I can answer is it should depend on the store you bought.
But, I see some flaws in your solution.
1. You need an older console, which it's more and more harder to find the first hand one.
According to reddit:
https://www.reddit.com/r/3dshacks/comments/50ovhy/new_3dsxl2ds_firmware_by_serial_september_2016/
The most FW you would found in the store is 10.7, which I'm not recommend because how the browserhax support this FW.
I have a fight with it 2 days ago until I give up and change exploit to OOT3Dhax. If you can't do game exploit, 10.7 is a pain in a bud.
2. If the console you found is a bundle one, such as Mario 3D Land N3DS. System transfer will transfer pre-load games to the one you want to Downgrade.
Which make it unlikely for the shop to accept return & refund.
3. If you're screwed up the DSi Downgrade process, you have to wait for another 1 week to do it again.
The more longer you need to wait, the more harder for the store to accept return & refund.
I think your best bet is to find second hand one with lower FW or already did A9LH, your friend's console. Otherwise, you need to go on with a hardmod.
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Veho:
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SylverReZ:
