Use an escrow agreement that places the purchase funds beyond their control, with release to the seller triggered by visa issuance and return to them only if the visa is refused. Supply the escrow agreement, deposit confirmation and the signed purchase agreement together.
Escrow is commitment with a safety catch
The Department of State accepts that funds placed in escrow with release conditioned solely on approval of the E-2 application are irrevocably committed for the purpose of the requirement, because the investor no longer controls them. Hypothetical example: the Lake Country couple deposit the full purchase price with an escrow agent under an agreement stating that the funds pass to the seller on visa issuance and revert to the couple only if the visa is denied; that structure is documented with the escrow agreement, the wire confirmations and the purchase agreement, and it commits the money without exposing them to total loss if the case fails.
What does not work is a refundable deposit that the couple can withdraw at will, or a promise to pay on closing with no funds moved. Beside the escrow, the file needs bank statements tracing the money from the Canadian property sale to the escrow account, the statement of adjustments from that sale and evidence of the couple's Canadian citizenship.