Having seen the example of Barnabas in 4.36-37, two among the believers named Ananias, with his wife Sapphira also wanted to be numbered among the generous. They, together, plan to sell a piece of property but only donate some of the proceeds to the community. We read that he kept back for himself some of the money and what Luke is saying is that Ananias implied and lied about this being the full amount (νοσφίζω, v.2).
Ananias and his wife wanted to seem more spiritual, more generous, more a part of the community than they actually were. They wanted to have the appearance of those who contributed generously, but were looking after their own interests first. As Peter said to Ananias, whilst the property remained unsold it was theirs to do with as they chose. Even after selling, the money was theirs to use as they saw fit (v.4). The sin is not not donating, but lying. Lying to the Holy Spirit in your heart that this is all the money you can give, lying to God (vv.3-4).
Confronted with his sin, Ananias fell down and breathed his last. Being confronted with his sin produced a violent and ultimately final reaction in him. He was happy enough, it seems, to lie and imply that this was all the money he could give, but when confronted with the truth, his sin has killed him (vv.5-6).
Perhaps you and I will never be in a position to sell a piece of property and donate it to a rapidly growing community that needs resources. However, the principle of wanting to appear more generous, more spiritual, more connected to the community by implying that we contribute more than we do is something that is not uncommon among us. When we do this, as Peter says, we are not only lying to ourselves and those around us, but to God. The Lord knows our intentions, our heart, and our motives, and we find ourselves sinning against Him when we lie or imply that our contributions are more than they really are.
As we think about Acts being prescriptive or descriptive, this passage in 5.1-6 stands as a description of what happened but also as a strong reminder that sins of lying and implying are more serious than most of us would care to think about.