No argument here. Fortunately for us, Oberyn more or less ignores his brother's will whenever it suits him, and right after we're done with Renly he's due to come to SD so we can help him prepare to raid the Rhoyne. Every single piece of loot they claim is one less for fucking Volantis.
Anyway, regarding that, things we have to offer them.
+1 weapons and armor commissioned from our mage-smiths
Silvered Weapons
Rhoynish Cold Iron mace that's been sitting unused in our treasury (he should be interested in this since it's a piece of his ancestors' history, and it's not like we had anyone lined up for it anyway)
Magelights
Obsidian Sand attuned to a brazier so that Tyene can talk to Oberyn pretty much whenever she feels like it (and so we can get a constant feed of intel during his mission)
An Arcane-marked stone so that we can teleport to him if he's in danger
Petty gear that can be crafted by Leila since we haven't specified her doing anything (Amulet of PfE)
He would be paying for everything except the magelights (although the silvered weapons are inexpensive enough that we can probably just gift that to him and gain some good will with Tyene).
@Goldfish, let us know if any of this panics you and you don't want to offer it.
EDIT: Oh, and a binding promise that he does
not in any way interact with the Mysterium.