JSFiddle - React, Tailwind, and code Playground

HTML

<input type="number" v-model="down_payment" class="form-control-right" placeholder="Downpayment" number>
<input type="number" v-model="loan_amount" placeholder="Loan Amount" number>

JavaScript

new Vue({
   el: 'body',
   data: {
     asking_price: 60000,
     down_payment: 20
   },
   computed: {
     loan_amount: {
       get: function() {
         return this.asking_price - this.down_payment;
       },
       set: function(newValue) {
         this.down_payment = this.asking_price - newValue;
       }
     }
   }
 });