Scheming and deception shows a lack of faith, this is what caused Abram and Sarai to go to Egypt and fool the Pharaoh, and this is why Sarai sought to fulfil God’s prophecy herself. She couldn’t believe that God would fulfil his promise that she would bear a son, and therefore she engineered the situation, giving her maid (an Egyptian maid was always going to be trouble) to her husband in the hope of the maid getting pregnant and carrying on Abram’s line. In the event she did get pregnant and gave birth to Ishmael, but this was not the fulfilment God intended, and therefore what they had engineered as a fulfilment ended up causing strife – and has done for millennia since, with the descendants of Ishmael seeking to wipe out the descendants of Israel for all this time.
The fact was, God had promised the pair a son, but he hadn’t said when that would be. They weren’t to wonder, they were to wait. That’s what obedience is, most of the time. With impatience comes disobedience, and with disobedience comes not taking God at his word. God said they would have a son; in time, Sarai and Abram stopped believing that – to the extent that they decided to work round it. As soon as you stop taking God at his literal word – you’re heading for trouble. Part of the problem was that Sarai and Abram were getting hung up on the baby part of the promise. Actually the baby was the smallest part of what God promised them – he promised them that they would be the head of a family that would become a nation, a nation that would be extraordinarily blessed by God. Circumstances needed to be right – and they needed to be right in such a way that Isaac’s birth would be recognised for what it was – a miracle. Obviously at the age of 85, Abram wasn’t quite ‘past it’ enough for God’s liking.
The Hagar thing was a scheme – and it was not God’s will, therefore it was a sin. Unfortunately sins have their consequences – and the consequences to this sin have been huuuuuge. First of all, because all three of them had taken their eyes off God, they were subject to their own petty emotions, they started to get all selfish, and were mean to each other. Petty grievances came out, ending in Hagar being banished, after Abram washed his hands of the whole matter. Everybody was blaming everyone else, no one was happy, the whole thing was a mess.
God met Hagar in the desert, and told her to return to Sarai and submit. This would have taken a reasonable amount of faith after Sarai had been horrible to her. She would have to return and submit, possibly to Sarai’s cruelty. God, in his grace, blessed Ishmael and made his descendants into a great nation – it is likely that many of the Arab peoples are descended from Ishmael. Unfortunately, the latter part of God’s prophecy has already come true that this people would be hostile, and would target the descendants of the true son (Isaac) in particular. This conflict remains today, and will remain until Jesus returns.
Yes, there were consequences to sin, but all involved were forgiven and restored to their rightful places. All of them in turn needed to submit to God, and to make the three-way relationship work, that surely is what they did.