Insufficient bitcoin funds error when sending RGB assets despite available BTC on other UTXOs

[berserker-systems]

Description

When sending an RGB asset, if the UTXO holding the asset doesn't have enough BTC to cover the witness output amount plus fees, the transaction fails with "Insufficient bitcoin funds" even when there's plenty of BTC available on other colorable UTXOs.

In this scenario, all BTC in the wallet was received via RGB transfers - no standalone BTC transactions were made to the wallet. This means all UTXOs are on the external (colorable) keychain, with no internal keychain UTXOs.

Steps to Reproduce

1. Create a wallet and receive an RGB asset with some BTC (e.g., 40,000 sats)
2. Receive another RGB asset (same or different) with some BTC (e.g., 60,000 sats)
3. Attempt to send one of the RGB assets with a witness amount larger than the BTC on that asset's UTXO (e.g., 70,000)

Expected Behavior

The transaction should succeed by using additional UTXOs as inputs to cover the required BTC amount.

Actual Behavior

Error: Insufficient bitcoin funds: needed '2000', available '0'``

The error reports 0 available even though there are ~100,000 sats in the wallet.

Note: The transaction succeeds when sending with 30,000 sats (less than the 40,000 sats available on the asset's UTXO), confirming the issue is specifically about not being able to use BTC from other UTXOs.

[nicbus]

This is more a design choice than a bug. Input selection doesn't consider colored UTXOs with RGB allocations as eligible. This is to avoid the unwanted consequence of merging lots of allocations into the same change UTXO, as it would easily exceed the max_allocations_per_utxo setting (especially considering the default UTXO size of 1000 sat). Note that unwanted consolidation of allocations on fewer UTXOs harms spendability while the transfer is pending and for future operations.

At the moment, the only solution is to create more colored UTXOs, as input selection will pick any colored UTXO with no allocation as additional input to cover BTC requirements.

We agree this is an annoying limitation though and are considering alternatives in order to give the user options other than having to create more colored UTXOs.