diff --git a/functions/src/buy-leaderboard-slot.tsx b/functions/src/buy-leaderboard-slot.tsx
new file mode 100644
index 00000000..2c63247c
--- /dev/null
+++ b/functions/src/buy-leaderboard-slot.tsx
@@ -0,0 +1,84 @@
+import * as functions from 'firebase-functions'
+import * as admin from 'firebase-admin'
+
+import { User } from '../../common/user'
+
+export const buyLeaderboardSlot = functions
+  .runWith({ minInstances: 1 })
+  .https.onCall(
+    async (
+      data: {
+        slotId: string
+        reassessValue: number
+      },
+      context
+    ) => {
+      const userId = context?.auth?.uid
+      if (!userId) return { status: 'error', message: 'Not authorized' }
+
+      // Run as transaction to prevent race conditions.
+      return await firestore.runTransaction(async (transaction) => {
+        const userDoc = firestore.doc(`users/${userId}`)
+        const userSnap = await transaction.get(userDoc)
+        if (!userSnap.exists)
+          return { status: 'error', message: 'User not found' }
+        const user = userSnap.data() as User
+
+        const { slotId, reassessValue } = data
+
+        // TODO: find most recent purchase of slotId.
+        // Fake data below:
+        const prevSlotPurchase = {
+          id: slotId,
+          reassessValue: 100,
+          userId: '',
+          timestamp: 0,
+        }
+
+        if (prevSlotPurchase) {
+          const prevSlotUserDoc = firestore.doc(
+            `users/${prevSlotPurchase.userId}`
+          )
+          const prevSlotUserSnap = await transaction.get(prevSlotUserDoc)
+          if (!prevSlotUserSnap.exists)
+            return { status: 'error', message: 'Previous slot owner not found' }
+          const prevSlotUser = prevSlotUserSnap.data() as User
+
+          const timeSinceLastPurchase = Date.now() - prevSlotPurchase.timestamp
+          const hoursSinceLastPurchase =
+            timeSinceLastPurchase / (1000 * 60 * 60)
+
+          const harbergerTax =
+            prevSlotPurchase.reassessValue * 0.1 * hoursSinceLastPurchase
+          const prevSlotUserBalance = prevSlotUser.balance - harbergerTax
+          if (!isFinite(prevSlotUserBalance)) {
+            throw new Error(
+              'Invalid user balance for previous slot owner ' +
+                prevSlotUser.username
+            )
+          }
+          transaction.update(prevSlotUserDoc, { balance: prevSlotUserBalance })
+        }
+
+        // TODO: If no prevSlotPurchase, use a default purchase price?
+        const newBalance = user.balance - prevSlotPurchase.reassessValue
+        if (!isFinite(newBalance)) {
+          throw new Error('Invalid user balance for ' + user.username)
+        }
+        transaction.update(userDoc, { balance: newBalance })
+
+        const newSlotPurchase = {
+          id: slotId,
+          reassessValue,
+          userId,
+          timestamp: Date.now(),
+        }
+
+        // TODO: save doc newSlotPurchase in some collection.
+
+        return { status: 'success', slotPurchase: newSlotPurchase }
+      })
+    }
+  )
+
+const firestore = admin.firestore()