Actually this refactor is better:

class PurchaseService {
  inProgress = false;

  constructor(
    private backend: PurchaseBackend,
    private messages: Messages) { }

  order(sku: string, qty: number): void {
    if (this.inProgress) {
      this.messages.error("Already purchasing an item");
      return;
    }

    let orderFromBackEnd = this.orderFromBackEnd.bind(this);
    let announcePurchase = this.announcePurchase.bind(this);

    this.backend.checkAvailability(sku, qty)
        .then(orderFromBackEnd)
        .then(announcePurchase);
  }

  orderFromBackEnd(enoughQuantity:boolean) {
    let finishProgress = () => this.inProgress = false;
    this.inProgress = enoughQuantity;
    return !enoughQuantity ? false :
           this.backend.order(sku,qty).finally(finishProgress);
  }

  announcePurchase((success: boolean) => {
    if (success) this.messages.success("Your purchase succeeded!");
    else         this.messages.error("Your purchase failed!");
  }
}