f liquidity check for offer-based amount

This commit uses the offer.amount() when amount_msats 
is not provided.

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::ln::wire::Encode;
 use crate::offers::invoice::{BlindedPayInfo, Bolt12Invoice, DEFAULT_RELATIVE_EXPIRY, DerivedSigningPubkey, ExplicitSigningPubkey, InvoiceBuilder, UnsignedBolt12Invoice};
 use crate::offers::invoice_error::InvoiceError;
 use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
-use crate::offers::offer::{Offer, OfferBuilder};
+use crate::offers::offer::{Offer, OfferBuilder, Amount};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
 use crate::onion_message::async_payments::{AsyncPaymentsMessage, HeldHtlcAvailable, ReleaseHeldHtlc, AsyncPaymentsMessageHandler};
@@ -8820,9 +8820,19 @@ where
 		let invoice_request = builder.build_and_sign()?;
 
 		let total_liquidity: u64 = self.list_usable_channels().iter().map(|channel| channel.next_outbound_htlc_limit_msat).sum();
-		if invoice_request.amount_msats() > Some(total_liquidity) {
-			log_error!(self.logger, "Insufficient liquidity for payment with payment id: {}", payment_id);
-			return Err(Bolt12SemanticError::InsufficientLiquidity);
+		let total_amount_msats = match invoice_request.amount_msats() {
+			Some(amount_msats) => Some(amount_msats),
+			None => match offer.amount() {
+				Some(Amount::Bitcoin { amount_msats }) => Some(amount_msats),
+				_ => None,
+			},
+		};
+
+		if let Some(amount) = total_amount_msats {
+			if amount > total_liquidity {
+				log_error!(self.logger, "Insufficient liquidity for payment with payment id: {}", payment_id);
+				return Err(Bolt12SemanticError::InsufficientLiquidity);
+			}
 		}
 
 		let context = OffersContext::OutboundPayment { payment_id };