atm.js

function atm (sum, limits) {
  const obj = {};
  let cur = 0;
  const arr = Object.keys( limits ).map(Number).sort((a, b) => b - a);
  let i = 0;
  while (i < arr.length) {
    const v = arr[i];
    const c = Math.floor( sum / v );
    const mc = Math.min( limits[ v ], c );
    i++;
    if (mc > 0) {
      cur += v * mc;
      sum -= v * mc;
      obj[v] = mc;
    } 
    if (!sum) {
      break;
    }
  }
  if (sum) {
    return 'Error';
  }
  return arr.reduce((result, key) => {
    if (obj[key]) {
      limits[key] -= obj[key];
      return result + obj[key] + 'x' + key + ' ';
    }
    return result;
  }, '').trim();
}