diff --git a/bitcoin/tx.c b/bitcoin/tx.c
index 40f5d9719..6995e302f 100644
--- a/bitcoin/tx.c
+++ b/bitcoin/tx.c
@@ -84,6 +84,41 @@ bool bitcoin_tx_check(const struct bitcoin_tx *tx)
 	return memeq(oldtx, tal_bytelen(oldtx), newtx, tal_bytelen(newtx));
 }
 
+void bitcoin_tx_output_set_amount(struct bitcoin_tx *tx, int outnum,
+				  struct amount_sat *amount)
+{
+	assert(outnum < tx->used_outputs);
+	tx->output[outnum].amount = *amount;
+	tx->wtx->outputs[outnum].satoshi = amount->satoshis; /* Raw: low-level helper */
+}
+
+void bitcoin_tx_input_set_witness(struct bitcoin_tx *tx, int innum,
+				  u8 **witness)
+{
+	struct wally_tx_witness_stack *stack = NULL;
+	size_t stack_size = tal_count(witness);
+
+	/* Free any lingering witness */
+	tal_free(tx->input[innum].witness);
+	tx->input[innum].witness = witness;
+
+	if (witness) {
+		wally_tx_witness_stack_init_alloc(stack_size, &stack);
+		for (size_t i = 0; i < stack_size; i++)
+			wally_tx_witness_stack_add(stack, witness[i],
+						   tal_bytelen(witness[i]));
+	}
+	wally_tx_set_input_witness(tx->wtx, innum, stack);
+	if (stack)
+		wally_tx_witness_stack_free(stack);
+}
+
+void bitcoin_tx_input_set_script(struct bitcoin_tx *tx, int innum, u8 *script)
+{
+	tx->input[innum].script = script;
+	wally_tx_set_input_script(tx->wtx, innum, script, tal_bytelen(script));
+}
+
 static void push_tx_input(const struct bitcoin_tx_input *input,
 			  const u8 *input_script,
 			  void (*push)(const void *, size_t, void *), void *pushp)
diff --git a/bitcoin/tx.h b/bitcoin/tx.h
index 3c15077a2..59b292ecb 100644
--- a/bitcoin/tx.h
+++ b/bitcoin/tx.h
@@ -92,6 +92,31 @@ int bitcoin_tx_add_input(struct bitcoin_tx *tx, const struct bitcoin_txid *txid,
 			 u32 outnum, u32 sequence,
 			 const struct amount_sat *amount, u8 *script);
 
+
+/**
+ * Set the output amount on the transaction.
+ *
+ * Allows changing the amount on the transaction output after it was set on
+ * creation. This is useful to grind a feerate or subtract the fee from an
+ * existing output.
+ */
+void bitcoin_tx_output_set_amount(struct bitcoin_tx *tx, int outnum,
+				  struct amount_sat *amount);
+
+/**
+ * Set the input witness.
+ *
+ * Given that we generate the witness after constructing the transaction
+ * itself, we need a way to attach a witness to an existing input.
+ */
+void bitcoin_tx_input_set_witness(struct bitcoin_tx *tx, int innum,
+				  u8 **witness);
+
+/**
+ * Set the input script on the given input.
+ */
+void bitcoin_tx_input_set_script(struct bitcoin_tx *tx, int innum, u8 *script);
+
 /**
  * Check a transaction for consistency.
  *