user34190: Is there a way to divide an unknown future input amount received by a given address amongst three recipients with proportions of, for example 0.2 , 0.3, 0.5?
This would need to be done in such a way that one could send an amount and be sure that it will be divided in this way.
It would not matter if the transaction was "blank cheque"/repeatable as described here.

user47827: No. You can't impose restrictions on where a current UTXO moves (without covenant functionality that isn't enabled on Bitcoin today) let alone impose restrictions on where a future UTXO moves.

user5406: As Michael already explained, it is not possible to make a transaction that spends unknown UTXOs since a transaction must state which outputs it consumes. Such a functionality could be partially enabled by e.g. sighash_anyprevout (BIP118) which identifies the spent input by the scriptPubKey rather than identifying a specific UTXO, but even then you'd need to know the expected value of the UTXO in advance, as the output amounts are predetermined.
A practical solution might be to run an automated process which regularly checks if there is a balance on the online service's wallet and then creates a transaction which splits the funds in the pre-defined proportions to the receivers. The receivers would then trust this service for the duration until the wallet is swept not to steal or compromise the funds.

34190: <p><strong>Is there a way to divide an unknown future input amount received by a given address amongst three recipients with proportions of, for example 0.2 , 0.3, 0.5?</strong></p>
<p>This would need to be done in such a way that one could send an amount and be sure that it will be divided in this way.</p>
<p>It would not matter if the transaction was &quot;blank cheque&quot;/repeatable <a href="https://bitcoin.stackexchange.com/a/13465/34190">as described here</a>.</p>
 47827: <p>No. You can't impose restrictions on where a current UTXO moves (without covenant functionality that isn't enabled on Bitcoin today) let alone impose restrictions on where a future UTXO moves.</p>
 5406: <p>As Michael already explained, it is not possible to make a transaction that spends unknown UTXOs since a transaction must state which outputs it consumes. Such a functionality could be partially enabled by e.g. sighash_anyprevout (BIP118) which identifies the spent input by the <code>scriptPubKey</code> rather than identifying a specific UTXO, but even then you'd need to know the expected value of the UTXO in advance, as the output amounts are predetermined.</p>
<p>A practical solution might be to run an automated process which regularly checks if there is a balance on the online service's wallet and then creates a transaction which splits the funds in the pre-defined proportions to the receivers. The receivers would then trust this service for the duration until the wallet is swept not to steal or compromise the funds.</p>
