Would appreciate any thoughts on this! Thanks!

(From release notes)
702.131a Assist is a static ability that modifies the rules of paying for the spell with assist (see rules 601.2g-h). If the total cost to cast a spell with assist includes generic mana, before you activate mana abilities while casting it, you may choose another player. That player has a chance to activate mana abilities. Once that player chooses not to activate any more mana abilities, you have a chance to activate mana abilities. Before you begin to pay the total cost of the spell, the player you chose may pay for any amount of the generic mana in the spell's total cost.

Since Battlebond cards were designed to be political in Commander, lets say Player A convinces Player B to assist him in casting a spell (for the sake of this discussion, lets just use Huddle Up). Player B agrees and taps his lands for mana. Player A now renegades, choosing not to pay the remaining cost for his own Huddle Up, and I believe the Huddle Up gets returned back to hand? Player A moves to combat step, lets Player B's mana drain, then casts the spell he really wanted to in Main Phase 2 knowing Player B has 2 fewer mana left to stop him.
Question:
Is this a legal way for Player A to trick Player B into tapping his mana sources at the wrong time? (Lame yes… but is it legal?)

Normally, we don't answer questions that involve as-yet-unreleased rules, because we don't have all the necessary information to do so–rules sometimes change, especially to account for new mechanics. In this particular case, however, I'm more than comfortable providing an answer: No.

In fact, more than just no: Hell no.

The rules for casting spells (CR 601) describe what happens if a player is unable to finish casting a spell, say because they're unable to pay its costs: the casting of the spell is illegal, and the game returns to the moment before the casting of that spell was proposed. This means that not only will the spell be back in its owner's hand, but the mana abilities Player B activated will also be undone. Player B's lands will be untapped, and they will have no mana floating.

On a more personal level, just as with the older Goblin Test Pilot + Suppression Field interaction, if a player deliberately tried this at a sanctioned event I was judging in, they'd be receiving a DQ for Cheating for their troubles.