The parable is about God's abundant generosity. Jesus used this parable to highlight the mercy of God, Who is the 'landowner', and all people are God's children. As God's children we all receive the same God's love. God discriminates against no one because God is love. We, humans discriminate against others, because we are driven by the world's standards.
The complaint was made, not because of the agreement on one denarius for a day's labour wasn't honoured, but because the expectation from the first group of labourers wasn't met. They raised the issue, because the last group of labourers got the same treatment as they did. They believed the last group of labourers received too much, for they had worked only an hour. For them it was unfair, and unacceptable, in the world's standard. The landowner, the Master, responded, that His generosity wasn't meant to be measured. Fairness, based on the labour market, shouldn't apply to God's generosity. the world's fairness, based on human greediness and vice, wasn't fair at all. The Master worked on a much higher level. It was the practice of heavenly virtues; the virtue of generosity. The Master challenged the workers: Are you envious because I am generous? Mat 20, 16. He told one of the complainants that they were in no position to get jealous. They were in no position to take control over who gets what, and how much the Master gave to the other workers. They were in no position to justify frustration with the Master, when He showed generosity, and certainly they were in no position to control the order of payment for the day work. The parable showed the first group of workers felt they were more important than the other workers. They should not use the worldly standard of justice to measure the heavenly standard of justice. For the Master, all workers were important, and all deserved to feel their worth.
The landowner agreed to pay the first group of workers one denarius for a day's labour. To the second and the third groups of workers, the landowner promised to 'give a fair wage'. We don't know what he meant by that, but they each received one denarius for their work. The last workers were employed without mentioning the amount of the payment. The first group of workers complained, that they worked hard all day, and endured the heat of the day. They failed to know, that they were blessed. They worked hard all day with peace of mind, that at the end of the day, they had a denarius to bring home. The other groups, all had endured the heat of the day as they were on standby, anxiously waiting to be hired. Do not fail to take into account the psychological anguish in waiting, worrying what would they bring home to feed their families. Their hope of being employed for the day dwindled as the day was going, and the prospect of being employed for that day was nil. Just before their hope died, the Master came to their rescue. There was no need of negotiating the payment, just something would be better than nothing. At the time of payment, to their surprise, they received much more than they could have imagined. The joy of receiving the Master's generosity overcame their anguish of waiting. They went home joyfully, chanting the Master's generosity. Their being patient to be employed paid off.