Calculate difference from latest value

This is one possibility:

let

// This function selects the corresponding B-Row for every A value
SelectB = (selection, date) =>

let
// several filters following
// omitt this first filter if you don't have the true/false-field
    #"Filtered Rows3" = Table.SelectRows(ChgType, each ([#"true/false"] = "true")),
// without the true/false-field, the next filter will look like this:
// #"Filtered Rows" = Table.SelectRows(ChgType, each ([Selection] = selection)),
    #"Filtered Rows" = Table.SelectRows(#"Filtered Rows3", each ([Selection] = selection)),
    #"Filtered Rows1" = Table.SelectRows(#"Filtered Rows", each [Date] <= date),
    #"Filtered Rows2" = Table.SelectRows(#"Filtered Rows1", each ([Type] = "B")),
// Just keep the latest row from this selection 
    #"Sorted Rows" = Table.Buffer(Table.Sort(#"Filtered Rows2",{{"Date", Order.Descending}})),
    #"Kept First Rows" = Table.FirstN(#"Sorted Rows",1)
in
    #"Kept First Rows",


// Example data - replace with your table
    Source1 = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjDUMzDTMzIwNFPSUVJwBBImIIZCBJAsKSpNVYrVgSgyRVFkhF2RCVyRE5Awxq7IGMUkU7CiSNyKnODWgRSlJeYUI1QZEWOfIYp9xtjtgysCGWSIpiYWAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Date = _t, Type = _t, Value = _t, Selection = _t, #"true/false" = _t]),
    ChgType = Table.Buffer(Table.TransformColumnTypes(Source1,{{"Date", type date}, {"Type", type text}, {"Value", Int64.Type}, {"Selection", type text}})),
// Here the function is called
    #"Added Custom" = Table.AddColumn(ChgType, "BValue", each SelectB([Selection], [Date])),
    #"Expanded BValue" = Table.ExpandTableColumn(#"Added Custom", "BValue", {"Value"}, {"ValueB"}),
    #"Added Custom1" = Table.AddColumn(#"Expanded BValue", "Difference", each if [Type]="A" then [Value]-[ValueB] else "")
in
    #"Added Custom1"