Avoid using mapTx.modify() to update modified fees

diff --git a/src/kernel/mempool_entry.h b/src/kernel/mempool_entry.h
index b274e1f7..3053a77c 100644
--- a/src/kernel/mempool_entry.h
+++ b/src/kernel/mempool_entry.h
@@ -80,7 +80,7 @@ private:
     const unsigned int entryHeight; //!< Chain height when entering the mempool
     const bool spendsCoinbase;      //!< keep track of transactions that spend a coinbase
     const int64_t sigOpCost;        //!< Total sigop cost
-    CAmount m_modified_fee;         //!< Used for determining the priority of the transaction for mining in a block
+    mutable CAmount m_modified_fee; //!< Used for determining the priority of the transaction for mining in a block
     mutable LockPoints lockPoints;  //!< Track the height and time at which tx was final
 
 public:
@@ -124,7 +124,7 @@ public:
     const LockPoints& GetLockPoints() const { return lockPoints; }
 
     // Updates the modified fees with descendants/ancestors.
-    void UpdateModifiedFee(CAmount fee_diff)
+    void UpdateModifiedFee(CAmount fee_diff) const
     {
         m_modified_fee = SaturatingAdd(m_modified_fee, fee_diff);
     }
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index ee4ced3b..de4a5572 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -598,7 +598,7 @@ void CTxMemPool::PrioritiseTransaction(const Txid& hash, const CAmount& nFeeDelt
         if (it != mapTx.end()) {
             // PrioritiseTransaction calls stack on previous ones. Set the new
             // transaction fee to be current modified fee + feedelta.
-            mapTx.modify(it, [&nFeeDelta](CTxMemPoolEntry& e) { e.UpdateModifiedFee(nFeeDelta); });
+            it->UpdateModifiedFee(nFeeDelta);
             m_txgraph->SetTransactionFee(*it, it->GetModifiedFee());
             ++nTransactionsUpdated;
         }
@@ -975,7 +975,7 @@ CTxMemPool::ChangeSet::TxHandle CTxMemPool::ChangeSet::StageAddition(const CTran
     TxGraph::Ref ref(m_pool->m_txgraph->AddTransaction(FeePerWeight(fee, GetSigOpsAdjustedWeight(GetTransactionWeight(*tx), sigops_cost, ::nBytesPerSigOp))));
     auto newit = m_to_add.emplace(std::move(ref), tx, fee, time, entry_height, entry_sequence, spends_coinbase, sigops_cost, lp).first;
     if (delta) {
-        m_to_add.modify(newit, [&delta](CTxMemPoolEntry& e) { e.UpdateModifiedFee(delta); });
+        newit->UpdateModifiedFee(delta);
         m_pool->m_txgraph->SetTransactionFee(*newit, newit->GetModifiedFee());
     }