Example of Subscribe to Property

your-component.component.ts

import {Component, OnDestroy, OnInit} from '@angular/core';
import {User} from './user'; // Assuming you have a class for user =====
import {Subscription} from 'rxjs/Subscription'; 
import {AngularFireDatabase, FirebaseListObservable, FirebaseObjectObservable} from 'angularfire2/database';

@Component({
  selector: 'your-component',
  template: `
    <h1>{{ (user | async)?.name }}</h1>
    <h1>{{ (user | async)?.price }}</h1>
    <p>
      {{ message }}
    </p>
    
    <button (click)="checkWallet()">
      Check Wallet
    </button>
  `
})
export class YourClass implements OnInit, OnDestroy {

  user: FirebaseObjectObservable<User>; // Your async user object
  userSub: Subscription; // Define this so you can unsubscribe when the component is destroyed
  
  myWallet: number;
  
  message: string;

  constructor(private db: AngularFireDatabase) {
  }

  ngOnInit(): void {
    this.user = this.db.object('user'); // Get the user object from firebase using AngularFire
  }

  checkWallet(price) {
    this.userSub = this.user.subscribe((user: User) => { // Subscribe to the FirebaseObjectObservable and assign the subscription to a property 
      if (user.price <= this.myWallet) {
        console.log(price);
        this.message = 'Yes I can buy the thing!!'
      } else {
        this.message = 'Tough luck buddy, you ain\'t got the cash'
      }
    });

    if (price <= this.myWallet) {
      console.log(price);
      //something
    }
  }

  ngOnDestroy(): void {
    this.userSub.unsubscribe(); // Use the Subscription to unsubscribe from the observable
  }
}